1. Field of the Invention
This invention relates to a liquid crystal material and a liquid crystal display. In particular, it relates to a liquid crystal material which can stably realize low resistance condition of a liquid crystal.
2. Description of the Prior Art
A display panel in an in-plane-switching (IPS) type of liquid crystal display where a liquid crystal is sandwiched between a pair of transparent substrates in a given distance has a feature that it can achieve a wider viewing angle than that in a display panel in a twist-nematic (TN) type of liquid crystal display where an electric field substantially vertical to a substrate surface is applied to a liquid crystal. It is because in the IPS type of liquid crystal display an electric field applied to the liquid crystal is substantially parallel to the substrates and thus the electric field rotates the liquid crystal molecules toward an in-substrate-plane and a horizontal directions.
An electric field substantially parallel to a substrate may be generated by disposing a pixel electrode and a common electrode, both of which are formed in a fashion of a comb-teeth form with a given space to each other on one of transparent substrates sandwiching a liquid crystal. An LCD utilizing such a lateral electric field has been proposed in, for example, Japanese Patent Publication No. 63-21907. Whereas a liquid crystal panel using the IPS system may achieve a very wide viewing angle, it has a problem that alignment disorder is apt to occur due to static electricity compared with a liquid crystal panel using a TN system because an opposite color filter substrate (the substrate where color filter is formed) opposing to a TFT substrate (the substrate where TFT is formed) does not have an ITO electrode.
A technique for preventing such an alignment disorder in an IPS type of liquid crystal display has been disclosed in, for example, Japanese Patent Laid-Open No. 7-306417, which is hereinafter referred to as xe2x80x9cconventional example 1xe2x80x9d.
Conventional example 1 has described that alignment disorder due to static electricity may occur easier because a liquid-crystal capacitance in the IPS system is smaller than that in the TN system. Specifically, when static electricity generates in a liquid crystal for some reason, the smaller the liquid crystal capacitance is, the larger an accumulated-charge difference of static electricity is between pixels with and without application of an electric field, leading to a larger luminance difference between them.
A smaller liquid crystal capacitance in the IPS system than the TN system is due to their different electric-field modes. Specifically, in the TN system an electric field is applied vertically to the substrates so that a liquid crystal between the upper and the lower substrates plays a role of capacitance, while in the IPS system an electric field is applied to a liquid crystal in parallel with a substrate in a relatively small distance near the surface of the one of the substrates so that the volume of liquid crystal to which the electric field is applied is less than that in the TN system, leading to a smaller liquid crystal capacitance in comparison with the TN system. For example, for a pixel size 300 xcexcmxc3x97100 xcexcm, it is 220 fF in the TN system while 100 fF in the IPS system.
Thus, there has been described that static electricity is prevented by reducing a specific resistance of a liquid crystal. There has been also described a technique that for achieving reduction in a specific resistance of a liquid crystal, the liquid crystal comprises a liquid crystal compound represented by formula (3) having at least one cyano, trifluoromethyl, trifluoromethoxy or nitro group as its end group: 
In formula (3) X1 to X3 represent fluoro, cyano, trifluoromethyl, trifluoromethoxy, nitro or hydrogen; R is optionally substituted alkyl or alkoxy having 1 to 10 carbon atoms; the ring A represents a cyclohexane, benzene, dioxane, pyrimidine or [2,2,2]-bicyclooctane ring; Z is a single bond, an ester linkage, an ether linkage, methylene, or ethylene; and n is an integer of 1 or 2.
The liquid crystal compound may have a molecular structure represented by formula (4) in which at least one cyano, trifluoromethyl, trifluoromethoxy or nitro group is introduced along the shorter molecular axis: 
In formula (4) X1 to X2 represent fluoro, cyano, trifluoromethyl, trifluoromethoxy, nitro or hydrogen; R is optionally substituted alkyl or alkoxy having 1 to 10 carbon atoms; the ring A represents a cyclohexane, benzene, dioxane, pyrimidine or [2,2,2]-bicyclooctane ring; Z is a single bond, an ester linkage, an ether linkage, methylene, methyleneoxy or ethylene; and n is an integer of 1 or 2.
In practice, a liquid crystal with a higher polarity does not always provide a specific resistance of 1013 xcexa9cm or less. Specifically, even a cyano-containing liquid crystal material may give a specific resistance of 1013 xcexa9cm or more, leading to alignment failure. Conventional example 2 (Japanese Patent Laid-Open No. 11-349948) has disclosed that a liquid crystal may advantageously comprise a dissociating substance represented by formula (5): 
In formula (5) R represents hydrogen, alkyl or alkoxy; X represents a cyclohexane or benzene ring; A represents phenyl substituted by at least one phthalic anhydride moiety, xe2x80x94COOH, xe2x80x94CONH, xe2x80x94NH2 and/or xe2x80x94OH; and n represents an integer of 1 or 2.
Conventional example 2 has described that the above problem can be solved by adding the dissociating substance in a liquid crystal. A dissociating substance as used herein refers to an acid- or base-dissociating substance, i.e., a substance spontaneously dissociating in a polar solvent to give H+ ions or reacting with water to give OHxe2x88x92 ions. It refers to, for example, a carboxylic acid including anhydride, amide, amine or alcohol. There has been described that such a substance may be added to a liquid crystal to increase an ion concentration in the liquid crystal and then reduce a specific resistance, resulting in the reduction of alignment failure due to static electricity.
The term xe2x80x9cpolarityxe2x80x9d as used herein refers to a state where in atomic or molecular level, a liquid crystal structure has atoms, e.g., N, O and F or functional groups, e.g., xe2x80x94COOH and xe2x80x94OH different in an electronegativity or electron density from each other. The term xe2x80x9ca higher polarity liquid crystalxe2x80x9d refers to that an electron density is higher along a longer axis of the liquid crystal molecule because of a particular functional group. It is thus believed that a higher polarity liquid crystal molecule withdraws impurity ions in the liquid crystal by its higher polarity functional group to reduce a specific resistance of the liquid crystal (more ions).
The term xe2x80x9cpolar solventxe2x80x9d refers to a solvent having a higher polarity functional group and a higher polarity solvent can easily dissolve a higher polarity molecule, while a lower polarity solvent can easily dissolve a lower polarity molecule. Therefore, when dissolving a dissociating substance in a higher polarity solvent in a liquid crystal, the dissociating ions are of a higher polarity.
However, an actual specific resistance of a liquid crystal varies even after enclosing it in a liquid crystal panel due to long-term operation of the liquid crystal panel or external temperature variation. An actual specific resistance in the liquid crystal may, therefore, vary suffering from various stresses even when the specific resistance is about 1013 xcexa9cm suitable for preventing alignment disorder due to static electricity immediately after enclosing the liquid crystal in the liquid panel. Specifically, when a specific resistance is increased, alignment disorder due to static electricity cannot be prevented while when it is excessively reduced, a voltage applied to the liquid crystal cannot be retained, leading to unevenness or spots in display.
After intense investigation, we have identified the causes for unevenness or spots and have found that a liquid crystal panel or liquid crystal material whose constitution will be described below may be used to prepare a highly reliable liquid panel.
There will be described a mechanism of variation in a specific resistance of a liquid crystal when adding a dissociating substance represented by formula (5), which in this case, has a carboxylic acid moiety.
A dissociating substance spontaneously dissociates in the liquid crystal to generate H+ ions, which are thus responsible for a major part of charge in the liquid crystal.
Furthermore, in the liquid crystal, a part of the dissociating substance dissociates to give a dissociation equilibrium state represented by formula (6). 
When a dissociating substance and its ion state after discharging an H+ ion are designated as BH and Bxe2x88x92, respectively, a dissociation equilibrium constant K is given by formula (7)                     K        =                                            [                              B                -                            ]                        ⁢                          xe2x80x83                        [                          H              +                        ]                                [            BH            ]                                              (        7        )            
where an expression with parentheses [ ] represents a concentration.
An H+ ion concentration [H+] in the liquid crystal dominating a specific resistance of a liquid crystal can be given by formula (8).                               [                      H            +                    ]                =                              K            ⁡                          [              BH              ]                                            [                          B              -                        ]                                              (        8        )            
On the other hand, a dissociation equilibrium constant K varies depending on the type of the dissociating substance and generally on a temperature. An H+ ion concentration, i.e., a specific resistance of a liquid crystal, also varies depending on a temperature.
There is an alignment layer defining an initial orientation of a liquid crystal as an uppermost layer of two transparent insulating substrates sandwiching the liquid crystal. The alignment layer adsorbs charge, i.e., H+ ions in the liquid crystal, leading to a change of a liquid-crystal specific resistance in the liquid crystal panel plane.
An objective of this invention is to provide a liquid crystal material which has a lower specific resistance considerably independent of a temperature and can reliably prevent alignment disorder due to static resistance evenly in a liquid crystal panel plane, as well as a liquid crystal display comprising the liquid crystal material.
This invention provides a liquid crystal material comprising a liquid crystal and an additive therein which comprises a dissociating substance and a conjugate salt thereof.
This invention also provides a liquid crystal display comprising a liquid crystal and a pair of substrates sandwiching the liquid crystal and controlling an orientation of the liquid crystal by a lateral electric field generated on one of the substrates, wherein the liquid crystal comprises a dissociating substance and a conjugate salt thereof.